The present invention relates to plastic blow molding machines.
A widely used process to produce containers, such as bottles, from a thermoplastic resin, such as polyethylene, is "blow molding". In that process plastic resin, generally in the form of pellets, is placed in a hopper which leads to an extrusion screw. The screw, by pressure or heat and pressure, causes the solid pellets to flow into a viscous semi-fluid mass of thermoplastic resin. The semi-fluid resin is forced by pressure to flow around a mandrel and through an extrusion die orifice which shapes it into a thin-walled elongated tube called a "parison".
The parison, having just been forced from the extruder die orifice and while still hot, is positioned over a blow tube and within an opened two-part mold. The mold is closed and its sealing members seal the top and the bottom of the parison to form an air-tight unit. Air is blown into the parison through the blow tube, causing it to expand against the inner walls of the mold. The walls of the mold are cooled and the cold walls of the mold cool the parison and form it into a bottle. The air is then exhausted and the two mold halves are opened.
This type of blow molding machine is generally described in U.S. Pat. No. 3,369,272 entitled "Apparatus For Concurrently Blow Molding And Trimming Plastic Articles" and U.S. Pat. No. 3,470,582 entitled "Apparatus For Compacting And Trimming Necks In Blow Molded Containers" describes a somewhat similar machine. Both of these patents are assigned on their faces to Hoover Ball and Bearing Company. In U.S. Pat. No. 3,369,272 the mandrel is elongated and has a bore containing the blow pipe. The top of the mandrel is threaded and screwed into a ring which is a portion of the die assembly.
In some blow molding machines a "pre-finish" neck is formed by a shearing action in the mold. When the mold parts are closed, a sealing and shearing member is moved into a cylindrical hollow on the blow pin, leaving a gap filled by the resin parison, to form a seal using the parison. Then the blow pin (sealing neck member) attached to the blow pipe is moved axially upwards; and the rising blow pin shears the plastic resin of the neck by forcing it against the shearing and sealing member which remains level. The blow pipe is raised and lowered within the bore of the hollow mandrel by an air cylinder piston. This type of blow molding machine is described in U.S. Pat. No. Re. 30,215 entitled "Sealing and Shearing Member In A Plastic Resin Blow Molding Machine" and in U.S. Pat. No. 4,234,299 entitled "Blow Pin Construction In A Plastic Bottle Blow Molding Machine."
In a typical blow molding machine, when the extrusion screw at its forward point hits a limit switch, a compressed air blast of low air pressure (for example, 45 pounds) for 2/10 to 3/10 of a second is sent through the blow pin to slightly blow (expand) the parison before the mold is closed. The mold is then closed and sealed and a blast of high pressure air (for example, 80 pounds) is expelled through the blow pin to expand the parison to the interior mold wall to form the bottle. The formed bottle is cooled by the cold mold wall for 3 to 5 seconds. The blow pin is then automatically pulled upwardly, for example, for a travel of 1/2 inch, to shear the bottle neck and form the round hole of the pre-finish neck.
An example of a blow molding machine is the UNILOY.TM. Model 350R2 made by Hoover Ball and Bearing Company. That machine may be equipped with four, six, eight, ten or twelve heads (molds). When equipped with six heads to make milk and juice gallon containers (less than 65-gram gallon size) it may make 2400 or more containers an hour.
In one type of blow molding machine the mandrel may be vertically lowered or raised relative to its forming die by means of an adjustment nut. The mandrel adjustment nut is freely rotatable within a plate and has interior screw threads which mate with the exterior screw threads on the top portion of the mandrel. A series of evenly spaced indentations are formed on the exposed upper portion of the mandrel adjustment nut. These indentations are round in cross-section and permit the insertion of a tool having a rounded boss. An operator may apply rotational pressure to the mandrel adjustment nut, turning it relative to the plate, by using the tool. As the mandrel adjustment nut is rotated on the screw threads of the mandrel, the mandrel is vertically lowered or lifted, depending upon the direction of rotation.
However, since there are vertical spacer pillars which prevent a full turn of the tool, it must be frequently repositioned in different indentations of the adjustment nut, preventing rapid and easy adjustment of the mandrel. When foreign particles become lodged in the extrusion orifice and the mandrel should be rapidly lowered, the slow action of the adjustment nut may cause a prolonged disruption in production. It has also been suggested that the adjustment nut may be rotated by a gear system. However, since the gear system presents a large mechanical advantage, the operator may not be sensitive to the orifice size and may injure the mandrel or the forming die.